Magnetic properties of a permanent magnet material, such as the known neodymium (Nd)-iron (Fe)-boron (B) permanent magnet alloy (e.g., Nd.sub.2 Fe.sub.14 B), can be altered by changing the alloy composition. For example, elements may be added to the alloy as substitution of existing alloying elements on the same lattice sites. More specifically, in the Nd--Fe--B alloy system, the magnetic properties can be altered by direct substitution of Fe, Nd and B by other elements at the Fe, Nd or B sites.
Magnetic properties of a magnetic material can also be altered by changing the microstructure of such alloy by changing the process conditions under which the alloy is made. For example, by rapid solidification, such as melt-spinning or atomization, it is possible to change the magnetic properties of such alloy by forming an extremely fine grain size directly from the melt or by over-quenching and then recrystallizing grains during a short time anneal.
Nd--Fe--B ribbons produced by the current industry practice of melt-spinning are known to exhibit both microstructure and magnetic property variations between the surface of the ribbons that touched the melt-spinning wheel and the free surface that did not touch the melt-spinning wheel, because of the differences in cooling rate across the ribbon thickness. Improvements in melt-spinning processes or products are therefore generally sought in two areas:(1) elimination of the inhomogeneities to yield better magnetic properties; or (2) increasing the production throughput while not further sacrificing homogeneity or properties. Current commercial production of Nd--Fe--B material by melt-spinning is limited to a throughput rate on the order of 0.5 kg per minute.
U.S. Pat. No. 4,919,732 describes melt-spinning a Nd--Fe--B melt to form rapidly-solidified flakes that include zirconium, tantalum, and/or titanium and boron in solid solution. The melt-spun flakes are then comminute to less than 60 mesh. They are subjected to a recrystallization heat treatment to precipitate diboride dispersoids for the purpose of stabilizing the fine grain structure against grain growth during subsequent elevated temperature magnet fabrication processes.
A disadvantage associated with use of precipitated diborides of hafnium (Hf), zirconium (Zr), tantalum (Ta), and/or titanium (Ti) to slow grain growth is the alloy competition between using the boron to form the boride and using the boron to form the tenary Nd--Fe--B 2-14-1 phase. This means that during alloying, extra boron is needed for compensating for this effect, which changes the location on the ternary Nd--Fe--B phase diagram and the resulting solidification sequence.
U.S. Pat. No. 5,486,240 describes a method for making a permanent magnet by rapidly solidifying a melt (of a rare earth permanent magnet alloy) to form particulates having a substantially amorphous (glass) structure or over-quenched microcrystalline structure. The melt has a base alloy composition comprising one or more rare earth elements, iron and/or cobalt, and boron. The alloy composition further comprises at least one of the following so-called transition metal elements (TM): Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and Al. The composition also includes at least one of carbon (C) and nitrogen (N) in substantially stoichiometric amounts with the transition metal TM to form a thermodynamically stable compound (e.g., transition metal carbide, nitride and/or carbonitride).
It is purported that the transition metal carbide, nitride and/or carbonitride compound is more thermodynamically stable than other compounds formable between the additives (i.e., TM, C and/or N) and the base alloy components (i.e., RE, Fe and/or Co, B) such that the base alloy composition is unchanged as a result of the presence of the additives in the melt. In one embodiment, the base alloy composition includes Nd.sub.2 Fe.sub.14 B, and elemental Ti and C and/or N provided in substantially stoichiometric amounts to form TiC and/or TiN precipitates.
It is disclosed in the '240 patent that the presence of the transition metal additive(s) (e.g. Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and Al) in the melt advantageously affects the glass forming behavior. That is, a much slower melt cooling rate can be used to achieve an amorphous structure. Thus, alloy component modifications (i.e., the amount of TM added) can be used to alter the glass forming ability to insure the desired amorphous structure is achieved in the rapidly solidified particulates.
However, there are several drawbacks associated with adding stoichiometric carbide, nitride and/or carbonitride to a Nd--Fe--B alloy. For example, it has been found that adding a large amount of compound forming elements (e.g., titanium and carbon) as a means of enhancing quenchability occurs at the expense of magnetic properties. There are two reasons for this: First, the added elements (e.g., titanium and carbon) form a separate nonmagnetic phase from the dominant Nd--Fe--B magnetic phase that dilutes the volume of the magnetic phase in the alloy. This is also called volume dilution.
Second, the added elements (e.g., titanium and carbon) poison the base Nd--Fe--B alloy, resulting in degraded magnetic properties. This effect is due to the fact that not all of the added elements (e.g., titanium and carbon) are used to form the compound (e.g., titanium carbide). Rather, there is always some solubility for the transition metal elements (e.g., Ti) in the 2-14-1 (Nd--Fe--B) phase (approximately 0.06 weight percent in the case of titanium), which effects magnetic properties, particularly magnetic remanence, B.sub.r, and maximum energy product, BHmax. In the case of Ti, for example, the negative effects of Ti substitution on the 2-14-1 phase properties are known to be significant.
Consequently, when adding stoichiometric amount of transition metal carbide or nitride (e.g., TiC) to achieve the desired levels of alloy quenchability, the combined reductions in magnetic properties attributable to volume dilution and poisoning of the 2-14-1 phase may render the magnetic properties commercially unacceptable. For example, the inventors of the present invention have shown that for a standard, commercially available Nd--Fe--B alloy composition, the optimum wheel speed used in melt-spinning (a direct measure of quenchability) for forming alloy powders may be reduced from about 20 meters-per-second down to about 8 meters-per-second by adding about three atomic-percent of TiC. However, the reduction in magnetic properties of the alloy appears to be more on the order of 20 to 30 percent, resulting in unacceptable properties, even though the amount of TiC second phase, which is nonmagnetic, comprises only about six volume-percent.
Moreover, it is believed that aluminum (Al) is mistakenly identified in the '240 patent as one of the so-called transition metal elements, because aluminum carbide, aluminum nitride, or aluminum carbonitride is not more thermodynamically stable than other compounds formable between the additives (i.e., TM, C and/or N) and the base alloy components (i.e., RE, Fe and/or Co, B). Thus, adding Al to the basic alloy in accordance with the '240 patent would not achieve the desired results.
It is therefore an object of the present invention to provide one or more additive elements and/or compounds to a base Nd--Fe--B compound to improve its quenchability;
It is another object of the present invention to minimize any degradation of the alloy magnetic properties caused by such addition of elements and/or compounds; and
It is a further object of the present invention to provide a method and apparatus for making such magnetic alloy at higher production through put than what has been possible in the past.